This invention relates generally to power driven hand tools and more specifically to a rotary impact wrench having an intermittent drive clutch mechanism.
Rotary impact wrenches of the type to which the present invention is related have employed different mechanisms for applying an impact force to an output shaft for turning a fastener element, such as a nut. These impacts develop relatively instantaneously high torque in the output shaft for tightening (or loosening) the fastener elements. Most rotary impact mechanisms include an output shaft formed in part as an anvil periodically impacted by hammers. The hammers are typically mounted for motion with respect to the anvil and a clutch mechanism is provided to periodically move the hammers between a position in which the hammers will strike the anvil, and a position in which they are clear of the anvil. When clear of the anvil, the hammers gain speed, and hence momentum, for the next impact with the anvil.
There are presently several types of impact mechanisms. One type of rotary impact wrench, such as shown in U.S. Pat. No. 3,661,217, uses a "swinging weight" mechanism in which hammer dogs are mounted for pivoting about axes parallel to, but spaced from the central axis of the output shaft. A lobe on the output shaft forms the anvil to be struck by the hammer dogs. The hammer dogs, which also rotate around the output shaft, periodically strike the anvil to deliver an impact to the output shaft. In another type of impact mechanism, a spring biases each hammer toward a position in which the hammer is in engagement with the anvil. However, cam balls riding in raceways in a motor driven shaft periodically force the hammers out of engagement with the anvil.
A third type of rotary impact wrench, such as shown in U.S. Pat. No. 2,881,884 and to which the present invention is particularly related, employs a "ski-jump" mechanism in which the output shaft is mounted for free rotation about its longitudinal axis in a tubular cage rotated by a motor about its longitudinal axis. The output shaft has two anvils projecting radially outward in opposite directions. Hammers mounted for rotation with the cage are spring biased axially away from the anvils, but connected to a cam follower for axial motion. A cam ball rotating with the cage periodically engages the cam follower, throwing the hammers forward into registration with the anvils so that they strike the anvils to deliver an impact force for turning the output shaft with a relatively instantaneous high torque.
Some of the prior "ski-jump" clutch mechanisms have taken the form of generally cylindrical pins which ride in generally U-shaped grooves formed at radially opposing positions in the internal wall of the cage. The grooves extend longitudinally of the cage to allow axial movement of the hammers in the grooves. The pins have narrow portions adjacent one end forming a circumferential recess or neck for receiving a portion of a cam follower therein. This interconnection transmits the axial motion of the cam follower in response to engagement with the cam ball to the pins to throw them into registration with the anvils on the output shaft.
The hammer pins fit relatively loosely in the channels so that upon impact with the anvils, there is some risk that the hammer pins will move radially out of the channels as well as laterally in the channels. This movement causes high stress at the neck of the pins, which may result in breakage of the pins at this location. Moreover, the radial and lateral movement of the hammer pins in their respective channels reduces the amount of pin surface area coming into registration with the anvils so that the impact tends to chip the pins and inefficiently transfer momentum to the anvils. The same problem occurs when the wrench is operated at higher than rated air pressures, which causes the hammer pins to rotate so rapidly that a smaller than designed length of the pins come into registration with the anvils before the pins impact the anvils. Moreover, the cylindrical shape of the pins allows for only a narrow line of surface contact between each anvil and pin. This small area of contact results in less efficient transfer of momentum from the pins to the anvils during the impact.